5 Size and Scale of the UniverseRealms of the UniverseImage courtesy of The Cosmic Perspective by Bennett, Donahue, Schneider, & Voit; Addison Wesley, 2002

6 Size and Scale of the UniverseEARTHPlanet where we all liveComprised primarily of rockSpherical in shape12,700 km in diameterIt would take 17 days to circumnavigate the globe driving a car at 100 km/hr (62 mph)At the speed of light, it would take 0.13 seconds to go all the way around EarthImage Credit: NASA/JPL/GSFC

7 Size and Scale of the UniversesunThe star that Earth orbitsComposed primarily of hydrogen and helium gasUses nuclear fusion in its core to generate heat and light to allow itself to resist the crushing weight of its own massSpherical in shape1.39 Million km in diameterImage Credit: SOHO/NASA/ESA

8 Size and Scale of the UniverseSun & EarthThe Sun’s diameter is 109 times greater than that of EarthOver 1 million Earths would fit inside the Sun’s volumeThe average distance between the Earth and the Sun is called an Astronomical Unit (AU) - it is about150 million kilometersIt would take 11,780 Earths lined up side to side to bridge the gap between Earth and Sun (or 107 Suns)Image Credit: SOHO/NASA/ESA

9 Size and Scale of the UniverseThe Solar systemImage credit: NASA8 planets, several dwarf planets, thousands of asteroids, and trillions of comets and meteoroidsMostly distributed in a flat diskPluto orbits ~40 AU from SunImage credit: NASA/JPL-Caltech/R. HurtThe Sun blows a constant wind of charged gas into interstellar space, called the Solar WindThe boundary between the Solar Wind and interstellar space (the Heliosphere) is around 100 AU from the Sun (200 AU diameter)Image credit: NASA

10 The Solar NeighborhoodSize and Scale of the UniverseThe Solar NeighborhoodThe region of the Galaxy within about 20 light-years of the Sun (40 light-years diameter)A light-year is the distance that light travels in one year (~10 trillion kilometers or 63,000 AU)The neighborhood stars generally move with the Sun in its orbit around the center of the GalaxyThe ‘Solar Neighborhood’ is a vague term not scientifically definedNote: the size of the stars in this image represents their brightness, they would actually all be specks at this distanceImage credit: Andrew Colvin

11 Size and Scale of the UniverseThe Milky Way GalaxyThe Milky Way Galaxy is a giant disk of stars 100,000 light-years across and 1,000 light-years thickThe Sun is located at the edge of a spiral arm, 30,000 light-years from the centerIt takes about 250 million years for the Sun to complete one orbitThere are over 200 billion stars in the Milky WayImage credit: R. Hurt (SSC), JPL-Caltech, NASA

12 The Local Group (of galaxies)Size and Scale of the UniverseThe Local Group (of galaxies)About 6.5 million light-years in diameterContains 3 large spiral galaxies -- Milky Way, Andromeda(M31), and Triangulum(M33) -- plus a few dozen dwarf galaxies with elliptical or irregular shapesGravitationally bound together—orbiting about a common center of massRoughly shaped like a footballImage Credit: Andrew Colvin

13 The Local SuperclusterSize and Scale of the UniverseThe Local SuperclusterThe Local Supercluster is about 130 million light-years acrossIt’s a huge cluster of thousands upon thousands of galaxiesLargest cluster is the Virgo cluster containing well over a thousand galaxiesClusters and groups of galaxies are gravitationally bound together, however the clusters and groups spread away from each other as the Universe expandsRoughly pancake shapedImage credit: Andrew Colvin

14 The Universe (the observable portion)Size and Scale of the UniverseThe Universe (the observable portion)Great walls and filaments of galaxy clusters surrounding voids containing no galaxiesProbably at least 100 billion galaxies in the UniverseSurveys of galaxies reveal a web-like or honeycomb structure to the UniverseImage Credit: G.L. Bryan, M. L. Norman, UIUC, NCSA, GC3Computer simulations also show a similar structure, often called the “Cosmic Web”Image Credit: Dr Chris Fluke, Centre for Astrophysics and Supercomputing, Swinburne University of Technology

15 The Universe (the observable portion)Size and Scale of the UniverseThe Universe (the observable portion)The Observable Universe is currently about billion light-years acrossThere could be (and likely is) much more beyond that, but we cannot see it from this point in spacetimeNote: The matter that we can see glowing shortly after the Big Bang (detected by the light it emitted 13.7 billion years ago) is now about 46 billion light-years away due to the ongoing expansion of the fabric of the UniverseImage Credit: Springer et al (2004)

16 So How do we know these distances?...Size and Scale of the UniverseSo How do we know these distances?...

17 Size and Scale of the UniverseThere are two basic methods for measuring astronomical distances: the standard rulers and the standard candles...1) The Standard RulersUse knowledge of physical and/or geometric properties of an object to relate an angular size with a physical size to determine distanceExamples: Parallax, Moving Clusters, Time Delays, Water MASERsConsidered to be a direct or absolute measurementRdd = R/Tan()  R/

18 …Parallax (a standard ruler)Size and Scale of the Universe…Parallax (a standard ruler)Image Credit: B. MendezRequires very precise measurements of stellar positions, and long baselinesNeed telescopes with high resolution, and must observe over several yearsThe Hipparchos satellite measured distances using this method for tens of thousands of stars within 1,500 light-years of the Sun

19 Size and Scale of the Universe2) The Standard CandlesUse knowledge of physical and/or empirical properties of an object to determine its Luminosity, which yields distance via the Inverse Square Law of LightExamples: Cepheid Variables, Supernovae, TRGB, Tully-FisherConsidered to be relative until tied to an absolute calibrationb = L/4d2Image credit: Splung.com

20 …Cepheid Variable Stars (Standard Candles)Size and Scale of the Universe…Cepheid Variable Stars (Standard Candles)Cepheid Variables are a type of giant star whose surface pulsates in and out with a regular period. That Period of pulsation is related to the Luminosity of the starImage credit: NASAThe Large Magellanic Cloud contains hundreds of Cepheids all at the same distance. Which allows for robust determination of the Period Luminosity RelationshipImage credit: NASA

21 …Supernovae (Type 1a) (Standard Candles)Size and Scale of the Universe…Supernovae (Type 1a) (Standard Candles)Supernovae are EXTREMELY BRIGHT explosions that can be seen from enormous distancesTheir absolute luminosity is known and fades at a consistent rate, so we can determine their distanceImage credit: David Hardy, PPARCWhite dwarfs capturing matter from a nearby star explode in special kind of Supernova called Type 1aType 1a supernovae are found by their spectral signatureImage credit: European Southern Observatory

22 Size and Scale of the UniverseThe Cosmic LadderTo measure cosmological distances a ladder of methods is used to reach furtherout into the Universe.Each “rung” in the ladder dependson the calibration of the methods “below” it.Image credit: Addison Wesley